1. Field of the Invention
This invention relates to an apparatus for use in a crystal manufacturing system based on a FZ (floating zone) method or CZ (Czochralski) method, the apparatus being adapted to control a quantity relating to the diameter of a crystal rod.
2. Description of the Prior Art
In the process of manufacturing a silicon semiconductor monocrystal, it is desired to minimize the length of conical portion, the first part of crystallization, which is not utilized as a product. However, as the length of this portion is reduced, crystallization is increasingly adversely affected into its disturbances due to requirement of steep change in the diameter. To prevent the occurrence of such disturbance, it is necessary to reduce the hunting to a level low enough during the operation of controlling the crystal diameter This consideration should also apply for elimination of surface irregularities of a main straight body portion of the crystal on comparatively a little softer level.
In a conventional method, therefore, a pattern (empirically determined) for supplying power to a heating device is previously written in a program setter, and the degree of deviation of the predetermined pattern from an ideal pattern (power supply pattern most suitable for obtaining a desired crystal diameter pattern) is minimized by a PI (proportion and integration) control or PID (proportion, integration and differentiation) control such that gains in the PI or PID control become smaller, that is, the proportional gain is reduced so as to lengthen the integration time while reducing the differentiation time, thereby limiting the amplitude of hunting.
In the FZ method, however, the discrepancy between the predetermined pattern and the ideal pattern becomes large so that the gains in the PID control are necessarily increased, because of different heating characteristics of various monocrystal manufacturing device of the kind and of those of even a single device due to a change with time or upon replacement of an induction heating coil. The above discrepancy further more, can be accelerated depending upon the chosen diameter of a polycrystalline rod, the manner in which the diameter of the conical portion changes or the varied length of a floating zone, from batch to batch of floating zone operation. There is therefore a great difficulty in reducing the hunting amplitude to a level satisfactorily low.
In the CZ method also, the discrepancy between the predetermined pattern and the ideal pattern becomes large so that the gains in the PID control are necessarily increased, because of different heating characteristics of various monocrystal manufacturing devices of its kind and of those of even a single device due to a change with time upon replacement of a resistance heater or crucible, or because the amount of silicon melt in the crucible decreases as the crystal grows Thus, there is a limitation on reduction in the amplitude of hunting in this case either.
Such problems are not limited to the case of silicon monocrystallization but it is common for them to exist with any monocrystal crystallizing technique based on the CZ or FZ method.